ON the face of it, there's no reason Patricia Cornwell shouldn't be the person to solve that greatest mystery in the annals of crime, the identity of the man who haunted the streets of London in the late summer and early fall of 1888, murdering and mutilating prostitutes and becoming known as Jack the Ripper. After all, Cornwell brings an array of talents and advantages to the undertaking: she has a keen investigative mind, as evidenced in her Kay Scarpetta mysteries, along with admirable empathy, as demonstrated in the work she has done for victims' and animal rights. She also brings the experience of having worked in a forensics lab, and of having taught herself the finer points of forensic investigation. Finally, she has more financial resources than most small or even moderate-size police forces could devote to the cause.

Sadly, Cornwell also brings an array of less fortunate characteristics to the Ripper case. First among these is a conviction that whatever unsupported conjectures she chooses to aggressively put forward carry the same weight as proven facts. Her belief that the efforts of all Ripperologists before her have been misguided is unsettling, too. ''I have avoided the recycled inaccuracies that have metastasized from one book to another,'' she declares -- a somewhat tasteless way of saying that she hasn't bothered to study thoroughly such scholars of the case as Donald Rumbelow, Martin Fido, Paul Begg and others. Worst of all, she gives pride of place to that great slayer of objective investigation, idiosyncratic emotional obsession. In this case this is all the more disturbing because Cornwell is obsessed with someone who is not only dead and unable to answer her many accusations, but who was an important creative figure in his own right and whose work is likely to be unfairly re-examined in light of her assertions.

The defendant in this case -- and this book is a prosecution, not an investigation -- is a man whose potential guilt has been considered by every responsible authority on the subject and dispensed with by nearly all of them. He is the post-Impressionist painter Walter Sickert (1860-1942), student of Whistler, colleague of Degas and guiding genius behind the Camden Town Group in the early 20th century. Sickert was one of the most important British painters of the half-century before World War II; he was also a failed actor with an intense interest in the macabre (especially grisly crimes) and a man who, though he had many intimate female friends and admirers (including Virginia Woolf, a fact of which Cornwell is apparently unaware), nonetheless had difficult relationships with at least two of his several wives.

Why has Cornwell zeroed in on Sickert, rather than any of the dozen or so other suspects who continue to be seriously weighed by experts in the Ripper case? That question is never adequately answered in this book, since from almost the very start Cornwell refers to Sickert and the Ripper interchangeably, taking away our ability and right to judge for ourselves. In interviews, Cornwell has been more forthright about why she picked Sickert. She has repeatedly said that she doesn't like the look of the painter's face in his self-portraits, or the subject matter and tone of his paintings generally.

The latter notion is, at least, not quite as psychologically projective a bit of detective work as is the former. Sickert did occasionally enjoy depicting crime scenes (he even rented rooms where he believed Jack the Ripper had once lived), and the female figures in a few of those sketches and paintings, as even some art historians will concede, bear a vague but unsettling resemblance to the mortuary and crime scene photographs of two of the Ripper's victims. Cornwell believes Sickert hadn't seen those photographs at the time he conceived his works, since they hadn't yet been published in England: therefore his paintings could only have been made possible by his having been present when the women died -- by his having been their murderer.

Here we run into an instance of Cornwell's ignorance of established research, an instance that is only the lead car in a long train of similar mistakes. Sickert summered nearly every year in France, where he had many friends -- and where the mortuary photographs of the two Ripper victims that he would eventually echo in his paintings were published in a popular crime book well before he created his sketches and canvases. How could Cornwell have missed this easily accessible fact? We may never know; and the sad truth is that the error is relatively minor compared to the misrepresentations that are to come.

The reason behind all the inaccuracies here soon becomes apparent: Cornwell confesses to never having given the Ripper case much thought before May of 2001. ''I had never read a Ripper book in my life,'' she writes. ''I knew nothing about his homicides.'' While she is perversely proud of this fact, the reader can only cringe: for it means that, at best, she has devoted all of 12 to 16 months to the single toughest serial killer case in history -- one could scarcely assimilate the available secondary sources, to say nothing of primary research materials, in that amount of time. She has cut corners and missed important points -- and just how many quickly becomes apparent.

Cornwell notes that as a boy Sickert underwent several surgeries to correct a ''fistula'' of some kind. If that fistula was penile, Cornwell posits, then the three painful surgeries could -- provided they were failures -- have left Sickert genitally deformed, impotent and incapable of having children, all of which would indeed be rich soil out of which to grow a frustrated sexual predator. Therefore, Cornwell simply decides that not only was Sickert's fistula in fact penile, but that the surgeries were bungled. Small matter that the surgeon who performed the procedures was an expert in rectal and venereal diseases, and that there is no record of his ever having performed surgery for a penile fistula on anyone. Even smaller matter that, later in life, one of Sickert's friends would scold him for fathering illegitimate children, or that at least one of those alleged children would eventually turn up. No, the penile deformity is postulated, and very soon it is treated for all practical purposes as a fact.

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Cornwell applies such tactics to many facets of the Ripper case. For example, she writes that ''handwriting is easy to disguise, especially if one is a brilliant artist.'' Sickert was a brilliant artist; therefore Cornwell considers herself free to speak of him as a brilliant forger, as well. And Cornwell has other insidious argumentative methods at her disposal. She employs the technique of guilt by accusation, in which the burden of proof is shifted from the attacker to the suspect, with mad abandon; and eventually she dispenses with even weak attempts at logical persuasion and begins to simply state possibilities as facts on the basis of nothing more than her intuition: ''It is quite possible,'' she writes, ''in fact, I would bet on it -- that Sickert showed up at every crime scene and blended into the crowds.'' Are we really supposed to accept Cornwell's willingness to bet on something as proof that it is true?

More of the critical aspects of the case receive similar treatment. Both Sickert's mother and a close French colleague wrote letters in September of 1888 indicating that Sickert was in France during the time that several of the Ripper murders took place. This is inconvenient for Cornwell -- so she declares it untrue: ''There is no evidence I could find that Sickert was in France at the end of September.'' Cornwell also creates trump cards to cope with such inconveniences: even if Sickert was in France in September, she says, he could have been commuting back to London -- to kill prostitutes.

CORNWELL wishes away other important theories, such as the possibility that there was a police and government cover-up and the tangential notion that a member of the royal family was involved in the killings. Yet perhaps more astounding even than her methods is the lack of command of several supporting subjects. She repeatedly displays confusion about blood identification, for example, castigating the coroners who attended to the Ripper's victims for not testing blood at the scenes of the killings and in other important spots. But in fact no test capable of determining whether a given blood sample was even human would be available until 1901. She harps on the idea that the Ripper was a psychopath, yet demonstrates a very tenuous understanding of the personality type and its behavioral manifestations. As for Cornwell's puerile statements about art, the less said the better. Her personal tastes make her so blind to Sickert's place in art history that she not only is mystified as to why the late Queen Mother should have bought one of his paintings and hung it in Clarence House, but is unable to see anything except clues to crimes in his work.

Why have Cornwell and her publisher made such inflated claims about her having solved the Ripper murders? The only concrete new element on offer here seems to be a series of DNA and paper tests that Cornwell and her team ran on some of the letters supposedly written by the Ripper to the police and newspapers, as well as tests on pieces of Sickert's own correspondence. Despite Cornwell's extraordinary statement that ''an imposing challenge for DNA experts is to help the hoi polloi understand what DNA is and what test results mean,'' we get the point here pretty quickly. Nuclear DNA -- the kind that's specific to one person -- was unobtainable from the samples tested, so the team had to switch over to mitochondrial DNA, which is only marginally better than blood typing in terms of narrowing things down. Using this test, Cornwell and her team discovered that some mitochondrial DNA on two of Sickert's letters matched some found on the backs of two stamps on a letter supposedly written by Jack the Ripper. In addition, Cornwell shows that the paper used in the same alleged Ripper letter was made by the company that produced the paper Sickert and his wife used for their stationery.

These facts are neither exciting nor decisive. Most of the Ripper letters were obvious hoaxes, and the one that Cornwell uses as her chief basis of comparison is notoriously suspect. But even assuming that the tested letter is real, we have narrowed things down only to the 1 percent of the British population that shared the same mitochondrial DNA sequences that can be found on the stamps. That is, it might have been Sickert who licked the stamps on the alleged Ripper letter, or it might have been any one of several hundred thousand other people. The situation regarding the paper brand is not much better: the company that made the letter's paper was one of the most popular manufacturers at the time, giving us a situation roughly analogous to what we would have faced in 1977 if one of David Berkowitz's famous ''Son of Sam'' letters to Jimmy Breslin had been written on a Hallmark card.

EVEN Cornwell can't work up too much excitement over all this: ''The Ripper case is not one to be conclusively solved by DNA or fingerprints,'' she concedes, ''and in a way, this is good. Society has come to expect the wizardry of forensic science to solve all crimes.'' She fails to point out that ''society'' has received a lot of help in that direction from people like herself.

''Portrait of a Killer'' is a sloppy book, insulting to both its target and its audience. The only way for Cornwell to repair its damage will be to stay with this case, as she says she intends to, continuing her research, studies and tests for the years required to complete them thoroughly. Perhaps then she can then do what she claims to have done already -- prove Walter Sickert's guilt decisively. Failing that, she should apologize for this exercise in calumny.

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A version of this review appears in print on December 15, 2002, on Page 7007015 of the National edition with the headline: Dealing With the Work of a Fiend. Today's Paper|Subscribe